O this thread is helpful. IME ottos are great diatom eater! I had diatoms in my tank and they cleaned them up in 2 days!
the trick in keeping ottos alive is to acclimate them properly and slowly!! I did the drip Acclimation! At about 1-2 drop per second until the water volume doubles , I then remove half the water the repeat the process but this time increasing drop rate to around 3-4 dps! The whole process took me over 3.5 hours and I also left the lights off for 3 hours after adding them into the tank. Ottos are also sensitive to ph fluctuation and high nitrate level. My 3 ottos has being in my 10 gallons with a male betta for more than 1 month now and they are doin well!